The invention relates to a method of reducing or preventing scale formation from aqueous solutions and also to an electrolysis apparatus for implementing this method.
The formation of scale from aqueous solutions is attributable mainly to the calcium and magnesium content of natural water and presents a general problem both in industrial and commercial operations and in the household since it causes a considerable need for maintenance and shortens the life of the appliances. To avoid these problems, it is usual to add chemicals which prevent scaling or to remove the substances which lead to scale formation, e.g. by means of ion-exchange processes, reverse osmosis, electrolysis and the like.
It is also known that the use of bipolar electrodes which can be configured as a fixed bed of conductive and nonconductive particles or as a fluidized bed of conductive particles makes possible higher space-time yields in electrolysis processes. Such processes are described, for example, in Electrochimica Acta 22, 347-352 (1977) and in Electrochimica Acta 22, 1087-1091 (1977) for preparing hypobromite, for the epoxidation of styrene, for the synthesis of the dimethyl ester of sebacic acid and for preparing hypochlorite from seawater. However, bipolar electrodes have not hitherto been used in water treatment.
Furthermore, GB-A-1 409 419 discloses a method of rendering pollutants such as chromic acid, cyanide or nitrate in aqueous solutions harmless by means of electrolysis using a bipolar fixed-bed electrode, in which there is added to the electrolyte a compound which reacts, or whose reaction product reacts, with the pollutant to form a compound which is not harmful. The bipolar electrode comprises nonmetallic, electrically conductive particles and can preferably also contain nonconductive particles.
An electrolysis cell having a bipolar fixed-bed electrode which can be opened at the upper end by means of a flap to allow easy replacement of the fixed bed is described in JP-A-04/027 491.
Electrochemical removal of contaminating ions from an aqueous medium is also described in U.S. Pat. No. 4,123,339, but this method uses electrodes made of iron or an insoluble iron compound and iron ions are released at the anode and hydroxide is formed at the cathode under the action of direct current, which is said to result in reaction with the contaminating ions to form an insoluble material which can be separated off. Uniform consumption of the electrodes can be achieved by reversal of the polarity.
U.S. Pat. No. 3,915,822 describes an electrochemical cell which, in the reaction zone, contains at least one bed of electrically conductive particles and has a plurality of electrodes which define adjacent chemical sections in which different voltage gradients can be maintained. The cell is said to be suitable for metal recovery, for adsorption and desorption of organic compounds, for oxidation of wastewater, for the synthesis of organic and inorganic compounds and the like.
DE-C-41 07 708 proposes a method of treating flowing water to prevent limescale by cavitation and an electric AC field, in which cavitation forms zones in which the pressure is significantly below the surrounding pressure in the water to be treated, which is said to result in local outgassing of the CO.sub.2 dissolved in the water and thus in a disturbance of the lime-carbon dioxide equilibrium and a decrease in the calcium solubility, and in which the water which has been treated in this way is passed between at least two electrodes. The use of electrodes having a structured surface, e.g. knobs, is said to make it possible to achieve the desired treatment effect at low voltages. However, it has been found in practice that the reduction in limescale which can be achieved by this method is barely significant.
Furthermore, EP-A-0 171 357 discloses an electrochemical method of softening water in which the alkaline pH in the vicinity of the cathode effects precipitation of the Ca and Mg ions in the form of their oxides and hydroxides which deposit on a porous inert material located between the electrodes. The porous material can be regenerated by reversing the polarity of the electrodes.
Since the known methods by means of which scale formation from aqueous solutions can be effectively prevented or reduced involve the addition of chemicals or the complete or substantial removal of the materials leading to scale formation they are often not very suitable, if at all, for water treatment. In particular, in the treatment of drinking water it is generally desirable for the natural salt content of the drinking water to be changed only slightly or not at all.